Hello all-
New to this forum, but not to the tube world.
My only real project has been building the "November" amp on the AX84 web site. I think it came out pretty good considering it was my first attempt, and built from scratch!

Aside from that, I fool mostly with tube radios.
I'm a novice in this field as I'm a mechanical designer by trade, so this electrical stuff is relatively new to me.

I recently aquired a McMartin MA-20 PA amplifier, and was wondering if any one knew anything about these amps?
My intention is to modify this amp for use with guitar. Looked to me like a good candidate since it has 2 6L6 output tubes.
I have a schematic coming in the mail, but I'm trying to gather more information before it get's here.
I'm sure I'll have a lot of questions as I'm going through this process, so bear with me.

Any information on these amps would be greatly appreciated!
Thanks!
Glenn

OK, I now have the schematic for this amp.
Can someone tell me what type of bias is used in this design?
I would like to change it to fixed bias for future ease of biasing the tubes. I plan on changing to KT66's.
Thanks
Glenn

Thanks for the link to the Goodrich site with McMartin schematics. My MA-50 is running pretty well now that I added 47uF to the output of the voltage regulator tube.

My favored hi-fi amps are a pair of Ampex 2012 (6V6GT outputs) pulled out of a 2-track Ampex reel-reel tape recorder. But that's getting off topic... <g> I was originally hoping the McMartin would be on par with the Ampex 2012, but being a PA amp instead of a hi-fi amp, it may not have a full frequency response. This sage advise came from mjd_tech. This weekend I'll complete my A-B tests, but the McMartin MA-50 will probably be best for guitar. But every time I putter with a tube amp, it's a great learning experience.

Very hard to see the drawing. But it appears to be a conventional fixed bias arrangement. Nothing esoteric in little PA amplifiers. The bias supply lookes like it has its own tap off the HV winding. it is a negative supply, half wave. The bias the tubes sees is determined by the voltage divider - that ratio of the two resistors in the supply. From the first filter cap there is a series resistor, and then another resistor to ground. The bias is taken at their junction, and fed to the output tube grids by the pair of resistors up there. They would be high value like 220k or something similar.

The two resistors in the supply form a voltage divider. The voltage out would be proportional to the resistors. Add them both, then use the grounded resistor to figure its percentage of the total. The bias is that percentage of the raw supply.

That is how it works, if the details vary, it is because I c an't see the picture.

To make the thing adjustable for differnt tube types, make the grounded resistor variable. The bias setup uses very little current, so you can use a range of values.

If you plan to use larger tubes, keep in mind they often use far more heater current than a 6L6 and the power transformer may or may not be up to it.

Enzo,
I'm not clear on what schematic you are talking about, nor what prompted the bias discussion and/or tube substitutions. Sorry if I seem dense today... must be the snow. Can you clarify which schematic you are talking about, and what prompted the bias discussion. It sounds interesting to a newbie like me, but I just need some more exposition.

Thanks,
Art

PS: my re-hab of my McMartin MA-50 went really well. My teenage son is now using it for his "big" guitar amp, driving a Peavey PA speaker that has a Black Widow driver for the woofer. It's working well and sounds awesome. My initial A-B tests against my Ampex 2012's didn't go well, but it turns out I had the bias set wrong. I need to do that again. Yes, I know PA amps are often not well suited for high fidelity, but most of the fun is learning that with your own two ears.

i have this mcmartin reciever its a one knob unit.
no tuner knob.
its a mystery?
it gets one fm station.104.0 fm.
the solid state circuit board number is 550179.
it says its a 5 watt amp/unit,
if you have any where online you can direct me to an answer or info on this unit .
thanks David